The present invention relates to a small motor and, more particularly, to a small motor having a commutator disposed in the aperture of a doughnut-shaped stationary magnet.
In general, the torque T acting on the rotor of a small motor is given by the following formula: EQU T=B.multidot.i.multidot.l
(where B represents the magnetic flux density of the stationary magnet, i represents the electric current in the coil and l represents the effective length of the coil perpendicular to the magnetic flux and the direction of rotation)
Therefore, the torque T can be increased by increasing one or more of the magnetic flux density B, current i and the coil effective length l. To increase the torque by increasing the effective length l, it may be considered to increase the diameter of the rotor as a whole or to make efficient use of a normally unutilized portion of the effective part of the coil adjacent to the commutator.
However, the former method increases the size of the motor as a whole. The latter method can be done, as shown in FIG. 1, by decreasing the inside diameter of a doughnut-shaped stationary magnet 7' from R.sub.1 to R.sub.2 to increase the effective length from l.sub.1 to l.sub.2 of the coil 6' so that the entire part of the effective length of the coil 6' is crossed by the magnetic flux. In this case, however, it is necessary to increase the distance between the stationary magnet and the coil because the end 6a' of the coil 6' undesirably contacts the stationary magnet 7' as a result of the reduction in the inside diameter of the magnet. Consequently, the magnetic flux density B is decreased, reducing and the torque cannot be sufficiently increased. In order to compensate for the reduction in the magnetic flux density B, is is possible to increase the wall thickness of the stationary magnet, but this increases the size of the motor as a whole.